The present invention relates to the reduction of sulfur-containing emissions (e.g., SO.sub.2 and SO.sub.3 gases, particulate sulfates, and H.sub.2 SO.sub.4 mist) from a large scale, continuous, flat glass melting operation. The term flat glass refers to glass commercially produced by the float process, plate rolling and grinding, and sheet drawing. Flat glass generally conforms to a relatively narrow composition range as follows:
SiO.sub.2 : 69-75% by weight PA0 Na.sub.2 O: 12-16% by weight PA0 K.sub.2 o: 0-2% by weight PA0 CaO: 8-12% be weight PA0 MgO: 2-5% by weight PA0 Al.sub.2 O.sub.3 : 0-2% by weight PA0 So.sub.3 : 0.15-0.5% by weight PA0 Fe.sub.2 O.sub.3 : 0-0.7% by weight
Commercial production of flat glass conventionally involves feeding raw glass batch materials into an opening at one end of an elongated melting furnace while withdrawing melted glass through an opening at the opposite end of the furnace and forming it into a continuous flat ribbon. Flat glass batches typically include sand (silica), soda ash (sodium carbonate), limestone (calcium carbonate), dolomite (calcium carbonate and magnesium carbonate), rouge (iron oxide), a source of sulfate such as salt cake, gypsum, etc., and sometimes the raw materials aplite, feldspar, or nepheline syenite. It is also known to use caustic soda in place of soda ash. Minor amounts of additional materials such as colorants may sometimes be used as well. These batch ingredients, in finely divided, dry, particulate form, are blended together and usually wetted with water prior to being introduced into the furnace. Additionally, a substantial amount of cullet (crushed glass) is mixed with the batch ingredients, in amounts usually ranging from about 20% to about 60% of the total glassmaking materials being fed to the furnace.
When introduced to the high temperature conditions within the melting furnace, the raw ingredients undergo chemical reactions and dissolution which, in a continuous flat glass furnace, normally take place within the first half of the furnace or less. The remainder of the furnace is devoted to "fining" (or "refining") and conditioning the glass melt. The process of fining is the removal of gaseous products of reaction from the melt by providing conditions which cause the gas bubbles to rise to the surface and burst or to redissolve in the glass. In a continuous glassmaking operation it is very important that conditions be maintained to enable fining of each portion of the melt to take place within its limited residence time in the fining zone of the furnace. Any gaseous inclusions which are carried out in the product stream form the defects known as "bubbles" (those having diameters larger than 0.25 mm.) or "seeds" (those having diameters smaller than 0.25 mm.) in the glass.
The problem of obtaining adequate fining is especially acute in a flat glassmaking operation since the standards for bubbles and seeds for flat glass are much more stringent than other types of glass such as bottle glass. For example, flat glass having one seed per square foot (0.09 square meter) would be considered rejectable for most flat glass applications, whereas what would be regarded as a very good grade of bottle glass may have on the order of 500 seeds per square foot (0.09 square meter) if formed into a sheet of the same thickness. In order to obtain adequate fining within a reasonable length of furnace, the flat glass industry has heretofore relied on the inclusion of large amounts of salt cake (sodium sulfate) and coal (carbon) in the batch as fining agents. The salt cake reacts to form substantial volumes of gaseous products which accelerate the movement of bubbles and seeds to the surface of the melt and help to homogenize the glass. Thus, it has long been the standard practice in the commercial production of flat glass to include substantial amounts of salt cake and coal in the batch ingredients fed to continuous melting furnaces. The customary use of salt cake and coal has also been based on other widely-held beliefs in the glassmaking art, such as the necessity for preventing "silica scum" (see "Handbook of Glass Maufacture," p. 66, F. V. Tooley, Ogden Publishing Co., N. Y., 1953) and for aiding the dissolution of sand grains (see Ceramic Bulletin, Vol. 54, No. 6 (1975), pp. 262-4).
Unfortunately, the use of salt cake as a fining agent has serious drawbacks. At glass melting temperatures salt cake dissociates or volatilizes, resulting in the emission of sulfur-containing gases. These may recombine with water vapor or sodium vapor within the furnace to form sulfuric acid mist or particulate sodium sulfate, which are not only air pollutants, but have a detrimental effect on the checker-packing in the regenerators of the furnace. Many widely varying proposals for reducing sulfur-containing emissions, have been made in the prior art, but none is entirely satisfactory.
One commonly proposed solution is to treat the effluent gas stream to remove the sulfur compounds. However, such an approach is costly and does not reduce the detrimental effects of the emissions on the regenerators. U.S. Pat. Nos. 3,788,832 and 3,880,639 disclose examples of the recovery and recycling of sulfur compounds from the exhaust gas stream by contacting the exhaust gas with incoming batch materials.
Elimination of salt cake is proposed in U.S. Pat. No. 3,846,143 by using in place thereof the reaction product of an alkali hydroxide and a source of alumina. It would be preferred that such an added pre-treatment step be avoided. Moreover, alkali hydroxides are generally a more costly batch material than salt cake and the seed counts reported in the patent appear to be much higher than permitted for flat glass. Substitution of SO.sub.2 gas for some or all of the salt cake as a source of sulfate is taught in U.S. Pat. No. 3,375,095 for the purpose of reducing deposition of sodium sulfate in the regenerators, but apparently without alleviating overall sulfur-containing emissions. Also, the use of SO.sub.2 gas as a sulfate source would usually be more costly.
Many other materials have been suggested for use as fining agents in addition to or as a substitute for salt cake, but without addressing the problem of sulfur emissions. In U.S. Pat. No. Re. 26,328 the use of calcium fluoride, gypsum, and slag are suggested as fining agents in a bottle glass operation. But gypsum and slag are both sulfur-containing, and the fluoride content of calcium fluoride can also be an air pollution problem. U.S. Pat. No. 3,589,885 discloses the use of carbonaceous material impregnated with a sulfate as a fining agent along with calcium fluoride for making bottle glass. U.S. Pat. No. 3,615,767 teaches the use of sodium sulfite as a fining agent. In U.S. Pat. No. 3,511,629 a frit containing sodium or barium sulfide is used as a fining agent. An article in "Ceramic Industry," Feb. 1972, page 31, suggests the use of elemental sulfur or slag in addition to salt cake as fining agents. The use of slag is also suggested in U.S. Pat. No. 3,725,022. None of these alternate fining agents is purported to alleviate sulfur-containing emissions problems, and for the most part appear to be merely substituting one source of sulfate for another in the batch.
It has also been previously suggested that pelletizing the batch as taught in U.S. Pat. Nos. 3,542,534 and 3,969,100 and the abovementioned U.S. Pat. No. 3,880,639, while intended primarily to ease melting, may have as a secondary advantage a lower salt cake requirement. The substantial capital investment and increased operating costs entailed by a pelletizing operation, however, make such an approach impractical in many cases. Moreover, since the presence of salt cake in these patents is said to be as a melting aid, and not as a fining agent, it is unclear what effect the reduction of salt cake would have on defect levels in a large scale, continuous, flat glass melting furnace employing pelletizing.
In U.S. Pat. No. 3,833,388 there is disclosed a glass composition differing from that of conventional flat glass, and which is said to require less salt cake, with a resulting reduction in sulfur-containing emissions. But because such a glass has properties which are slightly different from those of conventional flat glass, which properties are important in subsequent processing such as tempering, its use is preferably avoided.
Reduced amounts of salt cake are employed in the manufacture of one type of flat glass: colored glasses which incorporate selenium, cobalt, and nickel oxides, such as those disclosed in U.S. Pat. Nos. 3,296,004 and Re. 25,312. In glasses of this particular type, development of the desired coloration requires that oxidizing conditions be maintained and, therefore, salt cake and coal are minimized and alternate fining agents which act as oxidizing agents are employed, such as sodium nitrate or sodium chloride. The present invention, on the other hand, deals only with glasses which may be categorized as clear, or which contain iron oxide as the essential colorant. Since the refining agents used in the melting of the selenium, cobalt, and nickel colored glasses are more costly, and since the high oxidizing conditions are not required for conventional clear and iron tinted glass, the use of such alternate fining agents is preferably avoided when possible. Other alternate fining agents which are known in the art, but which are also preferably avoided, are arsenic oxide, antimony oxide, cerium oxide, and manganese oxide.
Other ways of reducing sulfur-containing emissions may be apparent to those of skill in the art, but each has serious drawbacks. For example, volatilization of salt cake may be reduced by lowering the melting furnace temperature, but the output of the furnace would be reduced and completeness of melting may suffer. Another possibility would be to reduce the amount of salt cake employed and compensate by increasing furnace temperatures. But the result would be shorter furnace life and greater fuel consumption. Yet another approach would be to increase the relative amount of cullet charged to the furnace along with the batch materials. This latter approach has been considered by some in the glass industry to be best solution to the emissions problem as evidenced by Business Week, Mar. 31, 1976, pp. 66B, 66H. But reliance on large amounts of cullet is preferably avoided because adequate supplies of suitable cullet are not always available in the flat glass industry, and excessive use of cullet represents inefficient utilization of a flat glass melting furnace in that more fuel is consumed to yield a net amount of glass. Thus, it would be desirable if sulfur-containing emissions could be reduced without altering the usual temperature conditions in a melting furnace, while at the same time using a high batch-to-cullet ratio.